To allow high-speed image data generation processing by shortening time required to generate an object having a predetermined gradation pattern over a number of lines in the vertical or horizontal direction in gradient fill object drawing processing, an image processing apparatus of this invention detects the number of lines of gradation in the horizontal or vertical direction, calculates only the first line of detected gradation, copies the calculation value of the first line, and embeds it in the second and subsequent lines, thereby shortening time required to generate objects of the second and subsequent lines and allowing high-speed generation processing.